This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. An essential step in retroviral infectivity is a process called "maturation", wherein the viral CA protein forms a closed capsid that acts to protect and organize the RNA genome. Mature retroviral capsids can be conical (HIV), spherical (M-MLV), or cylindrical (MPMV)(Vogt, 1997). Despite this variation in capsid morphology, the tertiary structure of all retroviral CA proteins is highly conserved, indicating that the different capsid shapes arise from common design principles. We have previously proposed that all retroviral capsids are built on hexameric lattices of CA and closed by incorporating pentameric declinations(Ganser et al., 1999;Li et al., 2000). The relative distribution of the declinations within the hexameric lattice defines the shape of the capsid. Two complementary model systems have been used by various groups to test this hypothesis. First, low-resolution biochemical and EM studies of in vitro assemblies have shown that full-length HIV-1 and retroviral CA proteins do form hexameric lattices, with the CA N-terminal domain (NTD) forming the hexamers, and the C-terminal domain (CTD) forming dimeric linkers that connect neighboring hexamers (Li et al., 2000;Mayo et al., 2002;Wilk et al., 2001). Second, x-ray crystallographic studies of truncated proteins have revealed a high-resolution model for the M-MLV hexameric domain, and several possible models for the CA dimerization domain(Ivanov et al., 2005;Jin et al., 1999;Mortuza et al., 2004;Worthylake et al., 1999). In addition, biochemical studies suggest the existence of NTD-CTD interactions essential for capsid formation(Ganser-Pornillos et al., 2004;Lanman et al., 2003). Therefore, a high-resolution structural model for full-length CA assemblies is required to unambiguously define protein-protein interfaces used by the NTD hexamer, determine the correct CTD dimer, and characterize the undefined NTD-CTD interface.